Electrophilic Addition to the Metal

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

An analogous ambiguity holds for nucleophilic reactions. We have already seen one facet of this problem in the oxidative addition of alkyl halides to metals (Section 6.3), which can go either by an electrophilic addition to the metal, the Sn2 process, or by SET and the intermediacy of radicals. The two processes can often give the same products. Other related cases we have s n are the promotion of migratory insertion and nucleophilic abstraction by SET oxidation of the metal (Sec. 7.1), and electrophilic abstraction of alkyl groups by halogen (Section 8.5). 

Oxidative addition by the Sn2 or ionic mechanisms involves two steps initial electrophilic addition to the metal (Eq. 8.29 and Sections 6.3 and 6.5), followed by substitution. 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

The extensive data accumulated by Nakamura and Otsuka, although interpreted by them as being due to the intervention of metal carbene and metallocyclobutane intermediates, can also be rationalized by an alternative mechanism in which coordination of the chiral Co(II) catalyst with the alkene activates the alkene for electrophilic addition to the diazo compound (Scheme 5.4). Subsequent ring closure can be envisioned to occur via a diazonium ion intermediate, without involving at any stage a metal carbene intermediate. 

This reaction does not involve an electrophilic addition to the alkene and, technically, does not belong in this chapter. However, it is convenient to include it here because it does result in addition to the alkene. The reaction occurs on the surface of the metal, so the catalyst must be present in a very finely divided or powdered state to maximize its surface area. A simplified version of the reaction mechanism on a platinum catalyst is presented in Figure 11.9. The unused valences of the atoms at the surface of the metal are used to break the hydrogen-hydrogen bonds, forming metal-hydrogen... 

Silver(I), Cu(II) and Hg(II) have been employed in the dimerization of 1 and discussed elsewhere as has the role of the first of these in the addition of alkenes, alkynes, allenes and conjugated dienes to cyclopropabenzene. It is more than probable that product formation with Ag(I) is dictated by electrophilic addition of the metal and subsequent interaction of the organosilver benzylic cation with the hydrocarbon reagent to give ring-opened or ring-expanded products. 

Examples of electrophilic interactions at the metal ions (as well as at the leaving and entering groups) have been demonstrated. These represent a formal oxidation of the metal and can lead to oxidative additions to the metal or, by transfer to a ligand, to a dissociation. On the other hand, at least some concerted oxidative additions, often described as electrophilic interactions, are seen in reality to resemble nucleophilic attack in their early stages and some of the two-step oxidative additions (most of which do conform to electrophilic attack) probably resemble the concerted mechanism in their initial interactions, the boundary being indistinct. Nucleophilic attack can, in some cases, precede electrophilic attack. 

Electrophilic substitution can be categorized into two subsets. One class of electrophilic substitution reaction involves electrophilic addition of the metal to the n system of an aromatic substrate, to form a Wheland-type intermediate, followed by proton loss (Scheme 11.35). This is the metal analogue of the classic organic... 

Complex 3 has been converted to a number of carbynes, disubstituted vinylidenes, and related complexes via electrophilic addition to the carbon of the vinylidene. - Complex 3 has also been used to build heterodinuclear transition metal complexes. ... 

There are two main classes of molecules (substrates) that can perform oxidative additions to metal centers non-electrophilic and electrophilic. Oxidative addition reactions with either class of substrates are favored by metal complexes that are more electron rich. Common non-electrophilic substrates are H2, Si-H bonds, P-H bonds, S-H bonds, B-H bonds, N-H bonds, S-S bonds, C-H bonds, alkenes, and alkynes. An important criterion for these non-electrophillic substrates is that they require a sterically accessible open coordination site on the metal (16e configuration or lower) onto which they need to pre-coordinate before initiating the oxidative addition to the metal center. For these substrates, both ligand atoms typically become cisoidally coordinated to the metal center after the oxidative addition as anionic (T-donors (subsequent ligand rearrangements, of course can occur). H2 is the most important and common for catalysis and a well-studied reaction is shown in Equation (5). 

The greater electrophilicity of coordinated arenes allows nucleophilic attack to occur on the arene unit. The steric effect of the metal fragment leads to attack on the side of the arene opposite to the metal, and this steric effect is used to control the relative stereochemistry of the substituents in the product of sequential reactions. Additions to the face of the arene bound to the metal can also be accomplished indirectly. In this case, the group is introduced by initial addition of a carbon electrophile, rather than nucleophile. The electrophile adds to the metal center, and this addition is generally followed by insertion of a coordinated CO and migration of the acyl group to the endo face of the arene. 

The pattern of chemical reactions observed for these compounds clearly sets them apart from "Fischer-type" carbyne complexes of GroupVI e.g., W(hCR)X(CO)4. Whereas the "Fischer-type" complexes typically react with nucleophiles at the carbyne carbon all of the reactions observed for the five coordinate mthenium and osmium complexes, including the cationic examples, are electrophilic additions to the MsC bond. The following sections deal individually with, protonation, addition of halides of the coinage metals, addition of chlorine and chalcogens, and finally an attempted nucleophilic addition where the nucleophile is directed to a remote site on the aryl ring of the carbyne substituent. 

Electrophilic addition to the sulfur atom of terminal thiocarbonyl ligands is a well-established route to thiocarbyne complexes of Group VI metals [2, 18, 19]. A requirement for the success of this approach is that the thiocarbonyl ligand be bound to a very electron-rich metal centre, preferably in an anionic complex. A methyltellurocarbyne complex of tungsten, [L(CO)2W=CTeMe]... 

CO is very sensitive to nucleophilic attack when coordinated to metal sites of low -IT basicity. On such a site, the CO carbon is positively charged because L-to-M a donation is not matched by L-to-M back donation and the CO ir orbitals are open to attack by the nucleophile. Alkyllithium reagents convert a number of metal carbonyls to the corresponding anionic acyls. The net negative charge now makes the acyl liable to electrophilic addition to the acyl oxygen to give the Fischer carbene complex, 8.1. ... 

In some cases the second step does not take place, and the counterion never binds to the metal. This makes the reaction an electrophilic addition, rather than an oxidative addition to the metal, although the latter term is sometimes seen in the literature to describe this type of reaction. An example is the reaction of the highly nucleophilic Co(I) anion, cobaloxime, with an alkyl triflate, a reaction known to go with inversion. Protonation of metal complexes to give metal hydrides is also very common (Eqs. 3.30-3.31). 

The reactions of organic free radicals with metal complexes is much less well understood than the attack of electrophiles and nucleophiles. If the starting material is an 18e complex, the product will be a 17e or 19e species and therefore reactive, so the nature of the initial reaction product may have to be inferred from the final products. Addition to the metal is well recognized and is easiest to detect when the starting complex is 17e, so that the product becomes 18e. For example, organic radicals are known to react very rapidly with [Co (dmg)2py] as follows ... 

There are a number of biological examples of halohydrin formation, particularly in marine organisms. As with halogenation (Section 8.2), halohydrin formation is carried out by haloperoxidases, which function by oxidizing Br or Cl ions to the corresponding HOBr or HOCl bonded to a metal atom in the enzyme. Electrophilic addition to the double bond of a substrate molecule then yields a bromonium or chloronium ion intermediate, and reaction with water gives the halohydrin. For example ... 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

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